As a mobile communications system develops, quality of service that can be provided by the system improves constantly. In order to further improve frequency domain efficiency and user throughput of the communications system, a carrier aggregation (Carrier Aggregation, Calif.) technology is introduced to LTE-A (Long Term Evolution-Advanced, long term evolution—advanced). In the carrier aggregation technology, a user equipment (User Equipment, UE) may use multiple serving cells simultaneously to perform uplink and downlink communication, so as to support high-speed data transmission. Among multiple serving cells that serve a same user equipment, one is called a primary cell (Primary Cell, PCell), and the other cells are called secondary cells (Secondary Cell, SCell).
In a communications system, in order to ensure that uplink data sent by a UE under a base station can arrive at the base station in a synchronized manner, in an uplink transmission process, the UE needs to maintain a timing advance (Timing Advance, TA), and the base station may adjust the timing advance for the UE according to an offset situation of an uplink signal. After the carrier aggregation technology is introduced, due to factors such as a location where a UE is or a technology adopted by a base station, timing advances of serving cells of a same UE may be different. In order to ensure that uplink data sent by a UE controlled by a base station can arrive at the base station in a synchronized manner, serving cells of the UE may be grouped according to whether timing advances are the same, and the timing advances of serving cells under a same timing advance group are the same, that is, the timing advance is of a group level.